linux-kernel - Re: [PATCH] delayacct: track delays from ksm cow

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Date:   Thu, 17 Mar 2022 11:05:22 +0100
From:   David Hildenbrand <david@...hat.com>
To:     CGEL <cgel.zte@...il.com>
Cc:     bsingharora@...il.com, akpm@...ux-foundation.org,
        yang.yang29@....com.cn, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] delayacct: track delays from ksm cow

On 17.03.22 10:48, CGEL wrote:
> On Thu, Mar 17, 2022 at 09:17:13AM +0100, David Hildenbrand wrote:
>> On 17.03.22 03:03, CGEL wrote:
>>> On Wed, Mar 16, 2022 at 03:56:23PM +0100, David Hildenbrand wrote:
>>>> On 16.03.22 14:34, cgel.zte@...il.com wrote:
>>>>> From: Yang Yang <yang.yang29@....com.cn>
>>>>>
>>>>> Delay accounting does not track the delay of ksm cow.  When tasks
>>>>> have many ksm pages, it may spend a amount of time waiting for ksm
>>>>> cow.
>>>>>
>>>>> To get the impact of tasks in ksm cow, measure the delay when ksm
>>>>> cow happens. This could help users to decide whether to user ksm
>>>>> or not.
>>>>>
>>>>> Also update tools/accounting/getdelays.c:
>>>>>
>>>>>     / # ./getdelays -dl -p 231
>>>>>     print delayacct stats ON
>>>>>     listen forever
>>>>>     PID     231
>>>>>
>>>>>     CPU             count     real total  virtual total    delay total  delay average
>>>>>                      6247     1859000000     2154070021     1674255063          0.268ms
>>>>>     IO              count    delay total  delay average
>>>>>                         0              0              0ms
>>>>>     SWAP            count    delay total  delay average
>>>>>                         0              0              0ms
>>>>>     RECLAIM         count    delay total  delay average
>>>>>                         0              0              0ms
>>>>>     THRASHING       count    delay total  delay average
>>>>>                         0              0              0ms
>>>>>     KSM             count    delay total  delay average
>>>>>                      3635      271567604              0ms
>>>>>
>>>>
>>>> TBH I'm not sure how particularly helpful this is and if we want this.
>>>>
>>> Thanks for replying.
>>>
>>> Users may use ksm by calling madvise(, , MADV_MERGEABLE) when they want
>>> save memory, it's a tradeoff by suffering delay on ksm cow. Users can
>>> get to know how much memory ksm saved by reading
>>> /sys/kernel/mm/ksm/pages_sharing, but they don't know what the costs of
>>> ksm cow delay, and this is important of some delay sensitive tasks. If
>>> users know both saved memory and ksm cow delay, they could better use
>>> madvise(, , MADV_MERGEABLE).
>>
>> But that happens after the effects, no?
>>
>> IOW a user already called madvise(, , MADV_MERGEABLE) and then gets the
>> results.
>>
> Image user are developing or porting their applications on experiment
> machine, they could takes those benchmark as feedback to adjust whether
> to use madvise(, , MADV_MERGEABLE) or it's range.

And why can't they run it with and without and observe performance using
existing metrics (or even application-specific metrics?)?


-- 
Thanks,

David / dhildenb